Heat the butter in a pan, add the onions and cook for 4-5 minutes until soft. Remove the onions from the heat, stir in remaining ingredients and set aside.
Place a sheet of fillo dough on a flat, clean surface and brush with sunflower oil. Cover with a second fillo sheet and brush with oil and repeat with a third fillo sheet. Cut the three fillo sheets into 9 squares and place 1 – 2 tbsp of the cheese mixture in the centre of each square. Holding the corners of each square, create a pouch, pinching at the neck so that it remains as closed as possible when baking.
Repeat 2 more times with remaining fillo and filling.
Place pouches on a baking sheet lined with greaseproof paper and bake in a preheated oven at 180°C for 20-25 minutes, until fillo has turned golden-brown. Let pouches cool for 5 minutes and serve.